1. Field of the Invention
This invention relates to field accessing of magnetic bubble domain arrays using contiguous element propagation patterns, and more particularly to a magnetic bubble domain storage system in which all bit positions of a contiguous element propagation pattern are occupied by bubble domains sufficiently close to one another that interactions occur therebetween.
2. Description of the Prior Art
Magnetic bubble domain technology has advanced considerably in the past five years. In particular, attempts are now being made to provide very high density storage systems using magnetic bubble domains for representing information. Two such techniques for achieving high density storage systems are the magnetic bubble lattice file, and the contiguous propagation pattern storage system. In the lattice file, bubble domains are packed very close to one another so that interactions exist between adjacent bubble domains. Since the bubble domains are packed so closely, lattice storage files have a higher storage density than conventional bubble domain storage systems where the bubbles are spaced sufficiently far apart that adjacent domains do not appreciably interact with one another. The basic concepts of a magnetic bubble lattice file are described in U.S. Pat. No. 4,052,710 assigned to the present assignee.
In the lattice file, information is coded in terms of the properties of the magnetic bubble domain system itself, since each bit position is occupied by a bubble domain. This type of coding is described in U.S. Pat. No. 3,890,605, also assigned to the present assignee. A technique for accessing information from selected columns of the bubble lattice file is described in U.S. Pat. No. 4,040,038, also assigned to the present assignee.
Bubble storage systems using contiguous element propagation patterns, commonly called "contiguous disk" patterns, do not pack bubble domains so closely to one another that appreciable interactions occur therebetween. Rather, the conventional bubble-bubble spacing of approximately four bubble diameters is followed to minimize interactions. However, contiguous disk bubble domain systems have an advantage over other types of bubble domain systems using isolated bubble domains in that the lithography requirements of the propagation elements are relaxed when contiguous elements are used. That is, the size of the individual propagation element in a contiguous disk pattern is large compared to the size of the bubble domain. This means that, for a given bubble domain size, the lithography required to make a contiguous disk propagation pattern is less stringent than that required to make discrete element propagation patterns, such as the familiar T and I-bar patterns or C-bar (half-disk) patterns. One of the reasons for this is that there is no gap across which bubble domains must propagate in a contiguous disk pattern, in contrast with the other patterns mentioned where bubble domains must cross a gap between adjacent propagation elements.
The conventional technique for moving magnetic bubble domains in a bubble lattice file uses current-carrying conductors to provide magnetic fields which move the bubble domains. However, the designs for doing this are complex and not all bubble domains in the lattice are directly driven (that is, bubble-bubble interaction is used to drive some of the bubble domains). Further, in-plane magnetic fields are often required for stabilization of wall states in the bubble domains in the lattice. The presence of the in-plane stabilization field seems to affect the propagation margins in the lattice, when it is moved by current-carrying conductor patterns. For this reason, field accessing of the lattice has been developed recently in order to improve propagation margins. Such techniques are represented by U.S. Pat. No. 4,023,150; U.S. Pat. No. 4,028,685; U.S. Pat. No. 4,034,357; and copending application to H. Chang, Ser. No. 645,594, filed Dec. 31, 1975 and now U.S. Pat. No. 4,067,002. In all of these references, arrays of magnetically soft propagation elements are used to move lattices of magnetic bubble domains in response to the reorientation of a magnetic field in the plane of the array.
Even though lattice translation by field accessing appears to offer better propagation margins, fabrication of arrays is still limited by lithography requirements. For example, the arrays typically comprise discrete propagation elements. In order to have the propagation elements directly act on all, or most, of the bubbles in the lattice, dense arrays have to be formed. However, this means that the propagation elements have to be close to one another and this requires advanced lithography. On the other hand, less dense arrays of propagation elements require less stringent lithography but also have smaller propagation margins, since increasing numbers of bubbles in the lattice are moved by the bubble-bubble interaction rather than by direct forces due to magnetic poles produced by the propagation elements.
In a conventional magnetic bubble domain lattice using current accessing techniques, translation of the lattice depends upon buffer regions produced at opposing ends of the lattice. These buffers usually contain stripe domains which shrink or elongate to cushion the lattice motion and maintain the integrity of the lattice. However, individual bubble domains are cut-off from the tips of the stripes to fill the voids in the lattice as the lattice is moved away from the elongating stripes. This cutting operation requires a high current density which may limit the bit density ultimately achievable with conventional bubble lattice files. Although this limitation is eliminated using the aforementioned field access techniques for translating a lattice, such field access lattices have more stringent resolution requirements than the conductor accessed lattices. For example, a permalloy propagation element in a field access system may have a width of approximately one-half bubble diameter.
Accordingly, it is a primary object of the present invention to provide a field accessed bubble lattice having reduced lithography requirements.
It is another object of the present invention to provide a contiguous element bubble propagation device having higher storage density than prior contiguous element propagation devices.
It is another object of the present invention to provide bubble lattice systems which exhibit the best features of both conventional contiguous element bubble systems and bubble lattice files.
It is another object of the present invention to provide a fully populated bubble device using contiguous element propagation patterns.
It is another object of the present invention to provide a bubble lattice system which requires less structure than conventional lattice systems and which has improved propagation margins.
It is yet another object of the present invention to provide ultra-high density contiguous element propagation storage devices.
It is another object of the present invention to provide a complete magnetic bubble domain storage system using contiguous element propagation patterns, where all bit positions in the storage system are occupied by interacting bubble domains.
It is a still further object of the present invention to provide improved field access bubble lattice systems.
It is another object of the present invention to provide a technique for moving bubble domains in an interacting array of domains using naturally occurring domain walls for high resolution propagation, rather than high resolution overlay or underlay structures.